In guidance, navigation, and control (GNC) systems, the attitude of a vehicle must be constantly monitored. Such vehicles include satellites, airplanes, unmanned aerial vehicles (UAVs), submarines, ICBNs, other aircraft, and other moving craft. At any given time, attitude control of such a vehicle depends upon precise measurement of its three dimensional angular changes (“pitch”, “roll”, and “yaw”) and its position relative to a frame of reference such as shown in FIG. 1. An error of a single degree could send a vehicle off course, several miles away from its intended trajectory. To ascertain their precise location, vehicles use a number of different on-board instruments. Some instruments are designed for relative (inertial motion) sensing and some instruments are designed for absolute (relative to a target) attitude sensing. For instance, to obtain a complete assessment of a satellite's attitude, navigation controls rely on inertial measurements (autonomous sensing), and absolute measurements (relative to a reference), both detected using two or more separate instruments.
A first class of devices is meant to detect the rate of change (rotation and acceleration) in the attitude of the vehicle, relative to its own inertial frame (autonomous sensing). Some of the most commonly used instruments in this detection system include “spinning mass”, “laser”, and “hemispherical resonator” gyroscopes, the latter being particularly expensive. The most common inertial navigation systems, however, use ring He—Ne lasers. This concept is more than 40 years old and is outdated. It has the following shortcomings: (i) a high power consumption/low efficiency, (ii) a lifetime limited by the vacuum tube technology, (iii) nonlinearity and zero response at low rotation rate (dead band), and (iv) configurations that occupy a large volume. Another commonly used system is the fiber optic gyro, FOG, which is a passive device that measures a phase shift through interference. In a FOG, the difference in phase induced by rotation (Sagnac effect) is translated into a difference in intensity. As a result, a FOG device can provide a considerably smaller signal to noise ratio and sensitivity than an active laser gyro based on frequency measurement.
A second class of instruments senses the position of an aircraft relative to outside objects. The most common instruments are: “horizon sensors”, “orbital gyrocompasses”, “sun sensors”, “star trackers”, and “magnetometers”.